Scientists image deeper melt reservoir at Yellowstone

A new reservoir of hot, partially molten rock located beneath the Yellowstone supervolcano has been imaged for the first time giving scientists a new perspective of the volcanic plumbing system under the supervolcano.

Scientists at the University of Utah, along with University of New Mexico Assistant Professor Brandon Schmandt, and others made the images from a newly-discovered, deeper magma reservoir located approximately 12 to 28 miles beneath the Yellowstone supervolcano.

The Yellowstone supervolcano is one of the largest active continental silicic volcanic fields in the world. An understanding of its properties is key to enhancing scientists’ knowledge of volcanic mechanisms and corresponding risk.

“For the first time, we have imaged the continuous volcanic plumbing system under Yellowstone,” said first author Hsin-Hua Huang, a postdoctoral researcher in geology and geophysics at the University of Utah. “That includes the upper crustal magma chamber we have seen previously plus a lower crustal magma reservoir that has never been imaged before and that connects the upper chamber to the Yellowstone hotspot plume below.”

The hot rock in the newly-discovered, deeper magma reservoir is nearly 4.5 times larger than the previously-known magma chamber, which scientists have studied for the past 10-15 years.

“In the past we’ve had strong evidence of a shallow magma chamber at about 5-15 kilometer depth and hot upper mantle at greater than 60 kilometer depth, but there was a big gap in our understanding of how melts from the mantle make their way upward through the lithosphere (tectonic plate) of North America,” Schmandt said. “The largest volume eruptions from Yellowstone are not composed of melted mantle, but rather more silica rich rocks that require a multiple stage process where mantle melts accumulate deep in the crust and mix with more silica rich rocks creating more buoyant melts.”

The 2,500-cubic mile upper magma chamber sits beneath Yellowstone’s 40-by-25-mile caldera, or giant crater. Scientists describe its shape like a gigantic frying pan about three to nine miles beneath the surface, with a “handle” rising to the northeast. The chamber is about 19 miles from northwest to southeast and 55 miles southwest to northeast. The handle is the shallowest, long part of the chamber that extends 10 miles northeast of the caldera.

A cross section of the magma chambers under the Yellowstone supervolcano. The larger, deeper chamber is the one that was recently discovered.

The new reservoir would fill the 1,000-cubic-mile Grand Canyon 11.2 times, while the previously known magma chamber would fill the Grand Canyon about 2.5 times.

“There is about one-quarter of a Grand Canyon worth of molten rock within the much larger volumes of either the magma chamber or the magma reservoir,” said Utah postdoctoral researcher Jamie Farrell, a co-author of the study.

Similar to past studies that made images of Yellowstone’s volcanic plumbing, the new study used seismic imaging, similar to a medical CT scan, but uses earthquake waves instead of X-rays to distinguish rock of various densities. Quake waves move faster through cold rock, and slower through hot and molten rock.

For the new study, Huang developed a technique to combine two kinds of seismic information: Data from local quakes detected in Utah, Idaho, the Teton Range and Yellowstone by the University of Utah Seismograph Stations and data from more distant quakes detected by the National Science Foundation-funded EarthScope array of seismometers, which was used to map the underground structure of the lower 48 states.

“It’s a technique combining local and distant earthquake data better to look at this lower crustal magma reservoir,” Huang says.

The Utah seismic network has closely spaced seismometers that are better at making images of the shallower crust beneath Yellowstone, while EarthScope’s seismometers are better at making images of deeper structures. Schmandt has been involved with EarthScope for several years.

Igneous, volcanic rock or rhyolites make up the biggest eruptions from Yellowstone. They are products of mixing and the creation of new melt composition.

“The basaltic volcanism represents mantle melts that ascended to the surface with relatively little interaction with the continental crust, which has a composition closer to granite than basalt,” Schmandt said. “The rhyolite volcanism is still fueled by heat from the mantle plume but its composition requires that large volumes of basaltic melt from the mantle accumulate and mix with the deep continental crust.”

Contrary to popular perception, the magma chamber and magma reservoir are not full of molten rock. Instead, the rock is hot, mostly solid and sponge-like, with pockets of molten rock within it. Huang says the new study indicates the upper magma chamber averages about nine percent molten rock – consistent with earlier estimates of five to 15 percent melt – and the lower magma reservoir is about two percent melt.

“The research could explain how a heat source in the mantle could eventually create large eruptions of lava that is more silica-rich than the mantle,” Schmandt said. “In oceanic settings, like Hawaii, melts have a shorter and more direct path from the mantle to the surface. At Yellowstone, the rocks indicate the need for a staging zone deep in the crust, but we haven’t been able to detect it well until now.”